Phosphors are used for a vacuum fluorescent display (VFD), a field emission display (FED or SED), a plasma display panel (PDP), a cathode ray tube (CRT), a white light-emitting diode (LED), and the like. In all these applications, it is necessary to provide energy for exciting the phosphors in order to cause emission from the phosphors. The phosphors are excited by an excitation source having a high energy, such as a vacuum ultraviolet ray, an ultraviolet ray, an electron beam, or a blue light to emit a visible light.
However, as a result of exposure of the phosphors to the above excitation source, there arises a problem of decrease of luminance of the phosphors due to a long term use and hence a phosphor exhibiting no decrease of luminance has been desired. Therefore, a sialon phosphor has been proposed as a phosphor exhibiting little decrease of luminance instead of conventional silicate phosphors, phosphate phosphors, aluminate phosphors, sulfide phosphors, and the like.
The sialon phosphor is produced by the production process outlined below. First, silicon nitride (Si3N4), aluminum nitride (AlN), calcium carbonate (CaCO3), and europium oxide (Eu2O3) are mixed in a predetermined molar ratio and the mixture is held at 1700° C. for 1 hour in nitrogen at 1 atmosphere (0.1 MPa) and is baked by a hot pressing process to produce the phosphor (See e.g., Patent Document 1).
The α-sialon activated with Eu ion obtained by the process is reported to be a phosphor which is excited by a blue light of 450 to 500 nm to emit a yellow light of 550 to 600 nm. However, in the applications of a white LED and a plasma display using an ultraviolet LED as an excitation source, phosphors emitting lights exhibiting not only yellow color but also orange color and red color have been desired. Moreover, in a white LED using a blue LED as an excitation source, phosphors emitting lights exhibiting orange color and red color have been desired in order to improve color-rendering properties.
As a phosphor emitting a light of red color, an inorganic substance (Ba2-xEuxSi5N8: x=0.14 to 1.16) obtained by activating a Ba2Si5N8 crystal with Eu has been reported in an academic literature before application of the present invention (See, e.g., Non-Patent Document 1).
Furthermore, in Chapter 2 of an academic literature “On new rare-earth doped M-Si—Al—O—N materials” (see, e.g., Non-Patent Document 2), a phosphor using a ternary nitride of an alkali metal and silicon having various compositions, MxSiyNz (M=Ca, Sr, Ba, Zn; x, y, and z represent various values) as a host has been reported.
Similarly, MxSiyNz:Eu (M=Ca, Sr, Ba, Zn; z=⅔x+ 4/3y) has been reported in Patent Document 2.
As another sialon, nitride or oxynitride phosphors, in Patent Documents 3 and 4 phosphors using MSi3N5, M2Si4N7, M4Si6N11, M9Si11N23, M16Si15O6N32, M13Si18Al12O18N36, MSi5Al2ON9, and M3Si5AlON10 (wherein M represents Ba, Ca, Sr or a rare earth element) as host crystals, which are activated with Eu or Ce, have been described. Among them, phosphors emitting a light of red color have been also reported. Moreover, LED lighting units using these phosphors are known.
Among them, there are known SrSiAL2O3N2:Eu2+ and Sr2Si4AlON7:Eu2+ as EuaSrbSicAldOeNf-based chemical compounds. Furthermore, Patent Document 5 has described a phosphor wherein an Sr2Si5N8 or SrSi7N10 crystal is activated with Ce.
Furthermore in Patent Document 6, there is a description of LxMyN(2/3x+4/3y):Z (L is a divalent element such as Ca, Sr, or Ba, M is a tetravalent element such as Si or Ge, Z is an activator such as Eu) phosphor and it describes that addition of a minute amount of Al exhibits an effect of suppressing afterglow. Moreover, a slightly reddish warm color white emitting apparatus is known, wherein the phosphor and a blue LED are combined.
Furthermore, Patent Document 7 describes phosphors constituted by various combinations of L Element, M Element, and Z Element as LxMyN(2/3x+4/3y);Z phosphors. Patent Document 8 describes a wide range of combinations regarding an L-M-N:Eu,Z system but there is not shown an effect of improving emission properties in the cases where a specific composition or crystal phase is used as a host.
In the representative phosphors in Patent Documents 2 to 8 mentioned above, phosphors using various different crystal phases as host crystals have been reported. The phosphors emitting a light of red color are also known but emitting luminance of red color is not sufficient by excitation with a blue visible light. Moreover, they are chemically unstable in some composition and thus their durability is problematic. Further, metal or nitride such as Ca or Sr is used as a starting material, so that it is necessary to mix powders in an outside air cutoff state, thus posing a problem in terms of productivity.